1. Nitrification and Denitrification with the Same Biomass
Biological nitrogen removal (BNR) is an important part of wastewater treatment and consists of two separate biological processes, nitrification and denitrification. These two processes have mutually exclusive requirements.
Nitrification is performed under aerobic conditions by slow-growing autotrophic nitrifying bacteria, using oxygen to convert ammonia to nitrite and then to nitrate. In contrast, denitrification is performed by heterotrophic bacteria that require anoxic conditions and a source of electrons, typically organic material (Chemical Omen Demand, COD) to convert nitrate into, nitrogen gas. The electrons are usually provided in the form of oxidisable organic and inorganic compounds in the wastewater.
A prime disadvantage of a single biomass is that the oxygen supply for nitrification generally causes the oxidation of some of the COD. As denitrification relies on COD as a source of electrons, electron donors often limit denitrification and additional donors must be added to the wastewater to compensate, adding to the cost of treatment. In addition, the slow growth rates of nitrifying bacteria render it difficult to maintain a sufficient proportion of nitrifying bacteria in a mixed microbial population. Lower numbers of nitrifying bacteria slow down the overall process substantially. Furthermore, high levels of organic matter and solids in wastewater lead to wash out of autotrophic nitrifiers. Some technologies with a single biomass use low oxygen concentrations to enable nitrification and denitrification to occur at the same time (simultaneous nitrification and denitrification). However this technology compromises both nitrification (inadequate oxygen supply) and denitrification (too much oxygen supply). It requires precise process control, which is difficult to establish because of the large diurnal fluctuations in wastewater flow. The above compromise exerts stresses on the nitrogen metabolism and has been suspected to cause problems by emitting N2O as an undesired greenhouse gas.
2. Separated Biomass (Dual Biomass)
The mutually excluding requirements for nitrification and denitrification have lead to attempts to separate the nitrifiers from the denitrifiers. The advantages of separating nitrifying and denitrifying biomasses for nutrient removal in wastewater treatment are numerous. Principal among these is the ability to customise the delivery of electron acceptors (such as oxygen and nitrate) and donors (such as ammonia and COD) specifically as required. This separation allows for optimised growth and performance of nitrifiers and denitrifiers with minimal waste of both oxygen and carbon sources, since the aerobic oxidation of carbon is avoided, helping to limit undesired biomass sludge formation. Separated biomass systems using entirely sequential operations of anoxic storage, aerobic nitrification and anoxic denitrification have been developed. However, these systems are limited in performance and cost by several intrinsic technical problems.
Nitrification reactors, regardless of design (trickling filter or sequencing batch reactor) produce a detrimental drop in pH, leading to a reduction in the activity of nitrifying bacteria. The continual need for pH control is known to be a significant cost factor in wastewater treatment.
Multireactor systems or sequentially operated dual biomass systems separate the nitrification biomass from the denitrification biomass. As a result, they require a holding tank to prevent mixing of the partially treated wastewaters between reactors, thus increasing their space requirements or “footprint”. The denitrifying biomass can be referred to as comprising a storage driven denitrification reactor, wherein the denitrifying biomass converts carbon to polymeric storage products to act as an energy store to fuel conversion of nitrates to gaseous nitrogen. Sequentially operated dual biomass systems are limited by the demonstrated physical retention of ammonia in the storage driven denitrification reactor during acetate uptake, which causes the release of untreated ammonia in the final reactor effluent from the denitrification phase. This is a major physical limitation to nitrogen removal efficiency such a system is able to accomplish, restricting it to a maximum nitrogen removal efficiency that corresponds to the amount of liquid that is retained in the biomass sludge after separation of bulk liquid from biomass. Accordingly traditional sludge settling with 50% settled sludge volume allows removal of only 50% of the nitrogen, whereas use of a sequential storage driven denitrification biofilm can allow about 80% nitrogen removal. To reduce ammonia retention, backwashing between the acetate (COD) uptake and denitrification phases can be used. This releases ammonia for treatment by the nitrification reactor. Such additional liquid flow may require additional holding tanks and increase the overall cast of operation.
It can be seen that the current waste treatment methods suffer from at least three main problems, namely pH drift, holding tank requirements and poor nitrogen removal capacity due to ammonia retention. The present invention addresses a need in the art for novel methods of removing nitrogen from liquids such as waste water.
The above discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.